Marketed in the United States as Truvada, and sometimes available abroad in generic versions, the pill has been shown to reduce the risk of getting HIV from sex by more than 90 percent if taken daily. Yet worldwide, only about a dozen countries have aggressive, government-backed programs to promote the pill. In the U.S., there are problems related to Truvada's high cost, lingering skepticism among some doctors, and low usage rates among black gays and bisexuals who have the highest rates of HIV infection.